Cantenna

A (can + antenna) is a homemade directional waveguide antenna, made out of a metal can or pringles can. The metal can is acting as a waveguide forces electromagnetic waves to travel in one direction.

Inside the can, RF energy bounces off the metal walls and forms patterns called standing waves.

Radiation Pattern

An antenna doesn’t send signal equally everywhere. It creates a radiation pattern — basically: where the signal goes and how strong it is in each direction

These patterns define an antenna’s coverage, focusing energy into specific shapes, to maximize efficiency in certain directions (directional) or all directions (omnidirectional).

Omnidirectional antennas

An omnidirectional antenna sends signal in all horizontal directions (360°).

like a shape of a donut or a light bulb shines in all directions, signal spreads around horizontally, very little goes straight up or down.

Lower range in any one direction and wastes energy sending signal everywhere.

Directional antennas

A directional antenna focuses signal in one specific direction. Like a beam or cone.

like a flashlight, focused beam in one direction.

Longer range. Stronger signal in one direction. Less interference.

Gain

Gain is how much the antenna focuses energy. Measured in dBi

Example:

Omnidirectional: 2-5 dBi
Directional: 8-20+ dBi

Higher gain = narrower beam + longer range

Beamwidth

The angle where most signal is concentrated

Type	Beamwidth	Coverage
Omni	~360°	Wide
Directional	10°-90°	Narrow

2.4 GHz Cantenna

Standard 2.4 GHz Wi-Fi runs at:

2.4 GHz = 2,400,000,000 cycles per second

Radio waves travel at the speed of light:

v ≈ 300,000,000 meters per second

Wavelength formula:

$$\lambda = \frac{v}{f}$$

$\lambda$ : The wavelength, usually in meters (m)
${v}$ : The speed of the wave, in meters per second (m/s)
${f}$ : The number of wave cycles per second, in Hertz (Hz)

$\lambda = \frac{300,000,000}{2,400,000,000} = 0.125$ meters

So a 2.4 GHz wave is about 12.5 cm long, but Wi-Fi is not one single frequency, the 2.4 GHz band actually ranges from: 2.412 GHz to 2.472 GHz

Each channel is slightly different, but let’s say the frequency is exactly 2.4 GHz then wavelength ≈ 12.5 cm

If frequency = 2.437 GHz (center channel 6)

Wavelength ≈ 12.3 cm

If frequency = 2.462 GHz (channel 11)

Wavelength ≈ 12.18 cm

So the difference between channels is only a few millimeters, so for practical cantenna builds use 12.3 cm as the working wavelength, it targets the middle of the band, it works well across all channels

after all at 2.4 GHz, a few millimeters won’t effect much.

The Probe

A small piece of copper wire connected to your Wi-Fi adapter. It sticks inside the can. Inside the can, waves reflect off the back wall.

You want the probe placed where the:

Electric field is strong
Energy transfer is efficient
Impedance is close to 50 ohms

That happens at roughly $\frac{1}{4}$ wavelength from the back wall

$\frac{1}{4}$ of 12.3 cm = $\frac{12.3}{4}$ = 3.075 cm

Antenna Polarization

Radio waves are electromagnetic waves.

They have:

An electric field (E-field)
A magnetic field (H-field)

Polarization is the direction the electric field oscillates.

If the electric field moves:

Up and down = Vertical polarization
Left and right = Horizontal polarization
Rotating = Circular polarization

If you rotate a vertical antenna 90°, it becomes horizontal.

antennas must match polarization.

If both antennas are:

Vertical = maximum signal
Horizontal = maximum signal
One vertical + one horizontal = major signal loss

Cross-polarization loss can be 20 dB or more

The polarization of a cantenna depends on the direction of the probe wire. If the probe is:

Vertical = vertical polarization
Horizontal = horizontal polarization

So if your router antennas are vertical your probe should also be vertical.

Parameter	Value
Frequency	2.4 GHz
Wave length	~12.3 cm
Probe length	3.0-3.1 cm
Probe distance from back	3.0-3.2 cm
Can diameter	7.5 - 10 cm
Minimum can length	12 cm (longer is fine)